Abstract: In order to investigate gas-liquid two-phase flow in horizontal mini circular tubes with inner diameters of 5.16 and 3.14 mm, a prism was adopted to improve the light path in the visualization experimental setup. The front and top views of air-water two-phase flow patterns in the two tubes were captured synchronously by the improved method. The effects of reflection on gas-liquid interfaces were eliminated with the help of Matlab. Three-dimensional gas-liquid interfaces, flow pattern maps and void fraction were obtained. The experimental results show that the main flow patterns in the mini tubes are bubbly flow, slug flow, and annular flow. Tube diameters have significant effects on flow pattern transition lines in the flow pattern maps, but the void fractions are independent of channel sizes. The effect of gravity gradually decreases with the decrease of tube diameter, while that of surface tension is enhanced. Therefore, the proportion of annular flow in the flow pattern map increases in mini tubes. The void fraction increases with the increase of gas quality. Experimental void fractions obtained using the three-dimensional gas-liquid interfaces fit well with correlations in the literature. Almost all of the experimental data are within ±25% error of the predicted values